Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 5-100 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a target material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range.
In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, for example in the form of a droplet, stream or cluster of material, with a laser beam. In this regard, CO2 lasers outputting light at middle infra-red wavelengths, i.e., wavelengths in the range of about 9.0 μm to 11.0 μm, may present certain advantages as a drive laser irradiating a target material in an LPP process. This may be especially true for certain target materials, for example, materials containing tin. One advantage may include the ability to produce a relatively high conversion efficiency between the drive laser input power and the output EUV power.
For LPP processes, the plasma is typically produced in a sealed vessel such as a vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from plasma formation such as source material vapor and/or clumps/microdroplets of source material that is not fully ionized in the plasma formation process.
Unfortunately, plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input optic, which may, for example, be a window or focusing lens.
The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them.
The use of a buffer gas such as hydrogen, helium, argon or combinations thereof has been suggested. The buffer gas may be present in the chamber during plasma production and may act to slow plasma created ions to reduce optic degradation and/or increase plasma efficiency. For example, a buffer gas pressure sufficient to reduce the ion energy of plasma generated ions to below about 100 eV before the ions reach the surface of an optic may be provided in the space between the plasma and optic.
In some implementations, the buffer gas may be introduced into the vacuum chamber and removed therefrom using one or more pumps. This may allow heat, vapor, cleaning reaction products and/or particles to be removed from the vacuum chamber. The exhausted gas may be discarded or, in some cases, the gas may be processed, e.g. filtered, cooled, etc. and reused. The buffer gas flows can also be used to direct particles away from critical surfaces such as the surface of the mirrors, lenses, windows, detectors, etc. In this regard, turbulent flows which can be characterized as having eddies, which can include fluid swirling and be accompanied by a reverse current, are undesirable because they may include flows that are directed toward a critical surface. These reverse current flows may increase surface deposits by transporting material to critical surfaces. Turbulent flows can also de-stabilize a target material droplet stream in a somewhat random manner. In general, this destabilization cannot be easily compensated for, and as a consequence, may adversely affect the ability of the light source to successfully irradiate relatively small target material droplets accurately.
Removal of deposits from optics in an LPP light source using one or more chemical species having a chemical activity with the deposited material have been suggested. For example, the use of halogen containing compounds such as bromides, chlorides, etc. has been disclosed. When tin is included in the plasma target material, one promising cleaning technique involves the use of hydrogen radicals to remove tin and tin-containing deposits from an optic. In one mechanism, hydrogen radicals combine with deposited tin forming a tin hydride vapor, which can then be removed from the vacuum chamber. However, the tin hydride vapor can decompose and redeposit tin if it is directed back toward the optic's surface, for example, by a reverse current generated by a turbulence eddy. This, in turn, implies that a reduced-turbulence flow (and if possible a laminar flow) that is directed away from the surface of an optic may reduce re-deposition by cleaning reaction product decomposition.
With the above in mind, Applicants disclose systems and methods for buffer gas flow stabilization in a laser produced plasma light source.